Caffeine is the most widely used stimulant in the world and one of the few where the dose people take is decided by habit rather than effect. Two numbers make it manageable: the daily ceiling and the half-life.
The daily ceiling
For healthy adults, guidance generally places up to about 400 mg a day in the range not linked with harm. Pregnancy guidance is substantially lower — commonly around 200 mg — and people with heart rhythm problems, anxiety disorders or certain medications may need less. Adolescents need far less again.
| Drink | Typical caffeine |
|---|---|
| Filter coffee, 240 ml | ~95 mg |
| Espresso, single | ~65 mg |
| Turkish coffee | ~60 mg |
| Black tea, 240 ml | ~45 mg |
| Green tea, 240 ml | ~30 mg |
| Energy drink, 250 ml | ~80 mg |
| Cola, 330 ml | ~35 mg |
The number that matters more: half-life
Caffeine's half-life averages about five hours. A 4 p.m. coffee of 95 mg still has roughly 47 mg circulating at 9 p.m. and around 24 mg at 2 a.m. That is enough to reduce deep sleep in many people — including those who insist they can "sleep fine after coffee". They usually can fall asleep; the sleep is just shallower.
The eight-hour rule
Set your caffeine cut-off eight hours before bed for two weeks and watch how the mornings change. It is the single most effective caffeine adjustment most people can make.
Why the first coffee often does nothing
Caffeine works by blocking adenosine, the molecule that builds sleep pressure. With regular use the body responds by making more adenosine receptors, so the same dose produces less effect — and skipping it produces a headache. That is tolerance and withdrawal, not a personality trait.
This is also why the morning coffee that "does nothing" is often just returning you to baseline rather than lifting you above it.
Kiva's suggestion
Delay the first coffee by an hour after waking. Many people find the same cup does noticeably more later in the morning.
Cutting back without the headache
- Count first. Two days of honest logging, including tea and cola. Most people are surprised.
- Reduce by about a quarter each week rather than quitting outright. Withdrawal headaches follow abrupt drops.
- Replace the ritual, not just the drink. The pause matters as much as the caffeine — decaf or herbal tea keeps the ritual intact.
- Fix the cause. Rising caffeine intake is usually a sleep-debt symptom. Treating the sleep resolves the demand.
What caffeine is genuinely good for
It reliably improves alertness, reaction time and endurance performance, and moderate coffee intake is associated with neutral-to-favourable long-term outcomes in large observational studies. The goal is not to eliminate it; it is to stop it from quietly eating your sleep.
Frequently asked questions
How much caffeine per day is safe?
Guidance for healthy adults generally puts up to about 400 mg a day — roughly four cups of filter coffee — in the range not associated with harm. Pregnancy guidance is lower, commonly around 200 mg, and should be confirmed with your doctor.
How long does caffeine stay in your system?
The half-life is around five hours on average, meaning half the dose is still circulating five hours later. Genetics, medication, smoking and pregnancy can move that figure substantially.
Does coffee dehydrate you?
Not at normal intakes. The mild diuretic effect does not outweigh the fluid in the drink, so coffee and tea count towards your daily fluid.
Keep reading
How much sleep do you actually need?
The honest answer to the 8-hour question, why sleep debt compounds, and what to fix first.
How much water do you need a day?
Where the eight-glasses rule came from, what really sets your needs, and the one check that beats every formula.